Transparent conducting film based on zinc oxide

Abstract

A transparent conducting film including a nominally undoped conducting ZnO base layer covered with a ZnO cover. The ZnO base layer has a preferred crystallographic orientation, whereas the ZnO cover includes one or more ZnO sublayers, of which at least one has a crystallographically randomly oriented or amorphous structure or a preferred crystallographic orientation different from the preferred crystallographic orientation of the base layer. The invention further relates to a process for the manufacture of such a transparent conducting film.

1 - 23 . (canceled) 
     
     
         24 . A transparent conducting film comprising a nominally undoped conducting ZnO base layer, the ZnO base layer having a preferred crystallographic orientation and being covered with a ZnO cover, the ZnO cover comprising one or more ZnO sublayers, of which at least one has a crystallographically randomly oriented or amorphous structure or has a preferred crystallographic orientation different from the preferred crystallographic orientation of the base layer. 
     
     
         25 . The transparent conducting film as claimed in  claim 24 , wherein the ZnO cover consists of a single ZnO sublayer having a crystallographically randomly oriented or amorphous structure. 
     
     
         26 . The transparent conducting film as claimed in  claim 24 , wherein the ZnO cover consists of a single ZnO sublayer having a single preferred crystallographic orientation. 
     
     
         27 . The transparent conducting film as claimed in  claim 24 , wherein the ZnO cover is a multilayer cover comprising plural ZnO sublayers, and wherein the ZnO sublayers of the ZnO cover have different preferred crystallographic orientations. 
     
     
         28 . The transparent conducting film as claimed in  claim 27 , wherein at least one of the ZnO sublayers of the ZnO cover has a crystallographically randomly oriented or amorphous structure. 
     
     
         29 . The transparent conducting film as claimed in  claim 27 , wherein at least one of the plural ZnO sublayers of the ZnO cover has a thickness comprised in the range from 2 nm to 40 nm and the ZnO cover has an overall thickness comprised in the range from 10 nm to 200 nm. 
     
     
         30 . The transparent conducting film as claimed in  claim 24 , wherein the ZnO cover is arranged in direct contact with the ZnO base layer. 
     
     
         31 . The transparent conducting film as claimed in  claim 24 , wherein the base layer surface has a normal, wherein the preferred crystallographic orientation of the ZnO base is (001) with respect to the normal. 
     
     
         32 . The transparent conducting film as claimed in  claim 24 , wherein the base layer surface has a normal, wherein the preferred crystallographic orientation of at least one of the ZnO sublayers of the ZnO cover is (110) or (101) with respect to the normal. 
     
     
         33 . The transparent conducting film as claimed in  claim 24 , comprising a flexible substrate carrying the ZnO base layer and the ZnO cover applied thereon. 
     
     
         34 . A semiconductor device comprising a transparent conducting film as claimed in  claim 24 . 
     
     
         35 . The semiconductor device as claimed in  claim 34 , wherein the semiconductor device comprises one or more of a transparent transistor, an array of transparent transistors, a flat panel display, an RFID chip, a photovoltaic cell and a capacitive sensor. 
     
     
         36 . The semiconductor device as claimed in  claim 34 , comprising a thin-film solar cell. 
     
     
         37 . A method of manufacturing a transparent conducting film comprising a nominally undoped conducting ZnO base layer, the ZnO base layer having a preferred crystallographic orientation and being covered with a ZnO cover, the ZnO cover comprising one or more ZnO sublayers, of which at least one has a crystallographically randomly oriented or amorphous structure or has a preferred crystallographic orientation different from the preferred crystallographic orientation of the base layer, the method comprising depositing a nominally undoped conducting ZnO base layer and a nominally undoped ZnO cover on the ZnO base by sputtering from a ZnO target in an inert gas atmosphere or from a Zn target in a mixed oxygen and inert gas atmosphere onto a substrate while maintaining a plasma close to the substrate; wherein the deposition of the ZnO base layer and the deposition of the ZnO cover are carried out with different densities of the plasma close to the substrate. 
     
     
         38 . The method as claimed in  claim 37 , wherein the plasma densities close to the substrate are chosen such that the ZnO base layer is deposited with a preferred crystallographic orientation and that the ZnO cover is deposited with a crystallographically randomly oriented or amorphous structure or with a preferred crystallographic orientation different from the preferred crystallographic orientation of the ZnO base layer. 
     
     
         39 . The method as claimed in  claim 37 , wherein the plasma density close to the substrate is maintained constant during the deposition of the ZnO base layer. 
     
     
         40 . The method as claimed in  claim 37 , wherein the plasma density close to the substrate is maintained constant during the deposition of the ZnO cover. 
     
     
         41 . The method as claimed in  claim 37 , wherein the plasma density close to the substrate is varied during the deposition of the ZnO cover. 
     
     
         42 . The method as claimed in  claim 41 , wherein the plasma density close to the substrate is maintained constant after one or more of the variations in such a way as to achieve a multilayer cover comprising plural ZnO sublayers with different crystallographic properties. 
     
     
         43 . The method as claimed in  claim 41 , wherein at least part of the variations of the plasma density close to the substrate are carried out with an amplitude and frequency such that a single preferred crystallographic orientation or a crystallographically randomly oriented or amorphous structure of the deposited ZnO results. 
     
     
         44 . The method as claimed in  claim 37 , wherein the plasma at the substrate is generated and maintained by RF biasing the substrate, and wherein the different plasma densities are obtained by modifying RF power density applied to the substrate.